Various implantable devices have been disclosed in the prior art that may be activated by ambulatory patients when the administration of measured doses of therapeutic agents is required. For example, cancer patients suffering from terminal lower torso cancer may require routine injections of morphine, either epidurally or intrathecally, and, upon receiving such injections, are sufficiently relieved of the symptoms of pain to move about and perform many routine and normal functions. Other chronic ailments also require frequent dosages of therapeutic agents in the treatment of chronic conditions, such as insulin in the case of diabetes. Implantable devices capable of delivering measured amounts of medicament on demand are disclosed in U.S. Pat. Nos. 4,634,427, 4,548,607, 4,588,394, 4,557,722, 4,544,371, and 4,543,088. Other U.S. patents of general interest pertaining to implantable pumping or infusing systems are U.S. Pat. Nos. 4,560,375, 4,258,711, 3,769,982, 3,827,439, 4,013,074, 4,265,241, 4,360,019, 4,487,603, 4,496,343, 4,511,355, 4,604,090, and 4,627,832. Reference may also be had to U.S. Statutory Invention Registration H150.